The subject of this study is the formation of the phase composition and structure in nanoscaled CoSbx (30 nm) (1.82≤x≤4.16) lms deposited by molecular-beam epitaxy on substrates of oxidized monocrystalline silicon at 200°C and the following thermal treatment in vacuum from 300-700 °C. It is established that after deposition, the lms are polycrystalline without texture. With increased Sb content, the formation of the phase composition in the lms takes place in such a sequence as is provided by the phase diagram for the bulk state of the Co-Sb system. With annealing in vacuum at temperatures above 450-500 °C, sublimation occurs not only for the crystalline Sb phase, but for the antimonides as well. This is reflected in the phase composition change by the following chemical reactions: CoSb2→ (600°C) Sb↑ = CoSb, CoSb3→ (600°C) Sb↑ = CoSb2, CoSb3+Sb↑→ (600°C) CoSb3 and leads to increases in the amounts of the CoSb and CoSb2 phases and decreases in the amounts of CoSb3. CoSbx(30 nm) (1.82≤x≤4.16) lms are found to be thermostable up to ≈350°C.